1. Field of Invention
The invention relates to an electrostatic actuator, preferably a micromachined or micro-electromechanical system (MEMS) based fluid drop ejector, having a segmented membrane with an independently addressable electrode for each segment.
2. Description of Related Art
Fluid ejectors have been developed for inkjet recording or printing, as well as other uses. Ink jet recording apparatus offer numerous benefits, including extremely quiet operation when recording, high speed printing, a high degree of freedom in ink selection, and the ability to use low-cost plain paper. The so-called “drop-on-demand” drive method, where ink is output only when required for recording, is now the conventional approach. The drop-on-demand drive method makes it unnecessary to recover ink not needed for recording.
Fluid ejectors for inkjet printing include one or more nozzles which allow the formation and control of small ink droplets to permit high resolution, resulting in the ability to print sharper characters with improved tonal resolution. In particular, drop-on-demand inkjet print heads are generally used for high resolution printers.
Drop-on-demand technology generally uses some type of pulse generator to form and eject drops. In one type of print head, a chamber having an ink nozzle may be fitted with a piezoelectric wall that is deformed when a voltage is applied. As a result of the deformation, the fluid is forced out of the nozzle orifice as a drop. The drop then impinges directly on an associated printing surface.
Another type of print head uses bubbles formed by heat pulses to force fluid out of the nozzle. The drops are separated from the ink supply when the bubbles form.
Yet another type of drop-on-demand print head incorporates an electrostatic actuator. This type of print head uses electrostatic force to eject the ink. Examples of such electrostatic print heads are disclosed in U.S. Pat. No. 5,534,900 to Ohno et al., U.S. Pat. No. 6,312,108 to Kato, U.S. Pat. No. 6,367,915 to Gooray et al., U.S. Pat. No. 6,409,311 to Gooray et al., U.S. Pat. No. 6,702,209 to Furlani et al., U.S. Pat. No. 6,572,218 to Gulvin et al., U.S. Pat. No. 6,357,865 to Kubby et al., U.S. Patent Application Publication No. US. 2002/0096488A1 to Gulvin et al., U.S. Patent Application Publication No. US. 2002/0097303A1 to Gulvin et al., and US. 2003/0087468A1 to Gulvin et al., the disclosures of which are hereby incorporated by reference herein in their entireties.
When ejecting fluid, such as ink, one typical requirement is the ability to modulate the drop size. For ink jet printing, this is used to change the amount of color that is provided at various points in a print output. This can also be done by altering the number of drops that land in a certain area, but that requires a much higher firing rate to achieve either a similar or a same print speed.
For mechanical drop ejectors, such as electrostatic and piezoelectric drop ejectors, the size of the drop is usually determined partially by the amount of displacement. For electrostatic drop elector membranes in particular, the membranes are partially pulled down and the difference in displaced volume of fluid leads to different size drops being ejected.
However, when voltage causes the membrane to be pulled down approximately ⅓ of the total distance between the electrodes, the exponentially increasing parallel-plate capacitive force becomes larger than the linearly-increasing elastic restoring force of the membrane. This gives rise to a well-known “pull-in” instability in which the membrane is snapped the remainder of the distance between it and the underlying electrode. Because of this, there is a gap in the range of useable displacements (e.g., effectively everything from ⅔ volume to full volume displacement becomes unusable). Thus, there is a gap in the size of drops that can be created.
The governing equation is derived below as equation (1), taking the derivative of the energy in the capacitor with respect to displacement to arrive at the force. The result is a function of both V and x, which is why the force increases so dramatically as the displacement increases.Fx=−∂U/∂x=−∂/∂x(½CV2)=−∂/∂x(½) (εoA/x)V2=(ε0A/2)(V/x)2  (1)
FIGS. 1–3 show a conventional MEMS-based electrostatically actuated diaphragm, in which a membrane is controlled by an electrode. FIG. 1 is a cross-sectional view of electrostatically actuated diaphragm 10 in a relaxed state. Substrate 20 is typically a silicon wafer. Insulator layer 30 is typically a thin film of silicon nitride, Si3N4. Conductor 40 acts as a counterelectrode and is typically either a metal or a doped semiconductor film such as polysilicon. Membrane 50 is made from a structural material such as polysilicon, as is typically used in a surface micromachining process. Nipple 52 is attached to a part of membrane 50 and acts to separate the membrane from the conductor when the membrane is pulled down towards the conductor under electrostatic attraction when a voltage or current, as indicated by power source P, is applied between the membrane and the conductor. Actuator chamber 54 between membrane 50 and substrate 20 can be formed using typical techniques, such as by surface micromachining.
FIG. 2 is a cross-sectional view of electrostatically actuated diaphragm 10, which has been displaced from its relaxed position by an application of a voltage or current between membrane 50 and conductor 40. The motion of membrane 50 then reduces the actuator chamber volume. Actuator chamber 54 can either be sealed at some pressure, or open to atmosphere to allow the air in the actuator chamber to escape (hole not shown). For gray scale printing, the membrane can be pulled down to an intermediate position using a charge driving mode. The volume reduction in the actuator chamber will later determine the volume of fluid displaced when a nozzle plate has been added as discussed below.
FIG. 3 shows a cross-sectional view of electrostatically actuated diaphragm 10, which has been pulled-down towards conductor 40. Nipple 52 on membrane 50 lands on insulating film 30 and acts to keep the membrane from contacting the conductor. This represents the maximum amount of volume reduction possible in the actuator chamber.
FIG. 4–6 show a conventional fluid ejector incorporating the diaphragm of FIGS. 1–3. FIG. 4 is a cross-sectional view of an electrostatically actuated fluid ejector 100. Nozzle plate 60 is located above electrostatically actuated membrane 50, forming a fluid pressure chamber 64 between the nozzle plate and the membrane. Nozzle plate 60 has nozzle 62 formed therein. Fluid 70 is fed into this chamber from a fluid reservoir (not shown). The fluid pressure chamber can be separated from the fluid reservoir by a check valve to restrict fluid flow from the fluid reservoir to the fluid pressure chamber. The membrane is initially pulled-down by an applied voltage or current. Fluid fills in the volume created by the membrane deflection.
FIG. 5 shows a cross-sectional view of the electrostatically actuated fluid ejector when the bias voltage or charge is eliminated. As the bias voltage or charge is eliminated, the membrane relaxes, increasing the pressure in the fluid pressure chamber. As the pressure increases, fluid 72 is forced out of the nozzle formed in the nozzle plate.
FIG. 6 is a cross-sectional view of the electrostatically actuated fluid ejector with the membrane back to its relaxed position. In the relaxed position, the membrane 50 has expelled a fluid drop 72 from pressure chamber 64. When the fluid ejector is used for marking, fluid drop 72 is directed towards a receiving medium (not shown).
The drop ejector uses deformable membrane 50 as an actuator. The membrane is typically formed using standard polysilicon surface micromachining, where the polysilicon structure that is to be released is deposited on a sacrificial layer that is finally removed. Electrostatic forces between deformable membrane 50 and conductor 40 deform the membrane. For constant volume or constant drop size fluid ejection, the membrane is actuated using a voltage drive mode, in which a constant bias voltage is applied between the parallel plate conductors that form the membrane and the conductor.
To avoid this “pull-in” instability, one could control the charge present instead of the voltage. Equation (2) below also differentiates the energy in the capacitor with respect to displacement, but it starts with the energy in terms of charge instead of voltage. As can be seen, the resulting force depends on charge, but not displacement. This solution balances capacitor force with membrane restoring force for all values of displacement, including those that are inaccessible using the above voltage drive mode. Examples of a fluid ejector driven in the charge mode can be founding U.S. Pat. No. 6,357,865 to Kubby et al. and U.S. Pat. No. 6,572,218 to Gulvin et al. Thus, for variable volume or drop size fluid ejection, the fluid ejector 100 relies on a charge drive mode, wherein the charge between the parallel plate conductors is controlled.Fx=−∂U/∂x=−∂/∂x(½)(Q2/C),=−∂/∂x(½) (x/ε0A)Q2=Q2/2ε0A)  (2)
Unfortunately, however, voltage drive mode is much easier to implement than charge drive mode, which requires a very complex circuit. One such circuit is a “switched capacitor” circuit, such as circuit 200 shown in FIG. 7 where a supply capacitor acts as the intermediary between the voltage source V and the load capacitor (e.g., the device or fluid ejector) so that they are never directly connected as would be in voltage control.
This device (fluid ejector) is used as the output capacitor of an op amp circuit in an integrator configuration. The supply capacitor is charged or discharged from voltage source V, then switched so that it connects to the load capacitor (e.g., the fluid ejector device), charging or discharging it. Charge is transferred to or from the load capacitor, and then the supply capacitor is switched back to the voltage source. This proceeds in cycles, which, if run quickly enough, start to resemble an analog waveform. However, because of this circuit complexity, voltage drive mode is much more prevalent than charge drive mode. Accordingly, conventional fluid ejectors have problems with generating variable drop size and suffer “pull-in” problems, unless complex charge drive mode electronics are implemented.